This invention pertains to a continuously variable hydromechanical transmission for a vehicle, and more particularly to a transmission having parallel axial piston pump and motor hydraulically linked through a stationary manifold and mechanically linked through a variable ratio gear set to provide an output torque with a constant mechanical portion and a variable hydraulic portion which diminishes to zero at hydraulic lock-up.
Interest in continuously variable hydromechanical transmissions has been increasing in recent years because of the potential operational efficiencies and economies that are increasingly becoming possible in vehicles and other powered systems wherein rotary input power is to be converted to output power at the desired output torque and speed. Continuously variable transmissions provide operational efficiencies and economies in the vehicle that are potentially superior to any known transmission, and theoretically can do so in packages that are smaller and lighter weight than other available transmissions.
However, conventional prior art hydrostatic transmissions are known by experts in the art to be noisy and inefficient. Convincing those experts and vehicle manufacturers that these new generation hydrostatic transmissions have overcome the intractable problems of the prior art is difficult. Therefore, additional improvements would facilitate acceptance of the new generation hydrostatic transmissions.
One such improvement would be in the area of leakage from rotating interfaces, particularly those where working fluid is commutated between the differentially rotating pump and motor.
Another improvement would be in the area of dynamic balancing. The difficulty of balancing rotating equipment to preclude vibration induced by rotating eccentric masses becomes worse exponentially with increasing speed of rotation. Yet another improvement would be in reducing the losses caused by xe2x80x9cwindagexe2x80x9d and fluid drag associated with the rotating elements inside the transmission housing. In applications having a prime mover with a high rotating speed, such as an electric motor, turbine engine or high performance spark ignition gasoline engine, the input elements would rotate at the prime mover output speed unless a gear reduction unit were interposed between the prime mover and the transmission. Gear reduction units add undesirable cost and weight. The windage and fluid drag losses can be greatly reduced by reducing the speed of rotation of those rotating elements. Still another desirable improvement would be in the area of manufacturability, simplicity, and cost. Prior art continuously variable hydromechanical transmissions have tended to be excessively complicated and costly to build. It would be a welcome development to original equipment manufacturers to have a continuously variable hydromechanical transmission available that is efficient, small and light weight, and is easily and economically manufactured and maintained.
One approach for achieving these improvements is shown in an international patent application No. PCT/US98/24053 filed on Nov. 12, 1998 by Folsom and Tucker entitled xe2x80x9cHydraulic Machinexe2x80x9d. A variation of this approach in a tandem hydromechanical transmission using low cost conventional components would make this technology available for smaller vehicles requiring more compactness and lower cost, such as outboard motors for boats, motor scooters, motor cycles, RV""s and snowmobiles.
Accordingly, it is an object of this invention to provide an improved hydromechanical continuously variable transmission for vehicles. Another object of this invention is to provide an improved method of transmitting power from a prime mover of a vehicle to the drive members of the vehicle (wheels, tracks, propeller, belt, etc) at output power in a continuously variable combination output torque and speed selected by the driver.
These and other objects are attained in a parallel hydromechanical continuously variable transmission having a housing holding a make-up pump and internal cavities for holding operating assemblies of the transmission, including an axial piston pump and an axial piston motor. The pump and motor each have a rotating element and a non-rotating element. Each non-rotating pump element is mounted for tilting movement in its own respective pair of mounting journals in the housing. The tilting axes of the non-rotating elements lie transverse to the axes of rotation of the rotating element. The pump and the motor are disposed side-by-side in the housing with the axes of rotation approximately parallel to each other. A variable ratio gear set couples the pump, motor, and output shaft so that the reaction torque from the pump is delivered directly to the output shaft. The pump and motor are coupled hydraulically through fluid passages in a stationary manifold, fixed in the housing. Internal fluid passages in the stationary manifold convey fluid pressurized in the pump directly to the motor, and convey spent fluid displaced from the motor back to the pump. The transmission ratio is controlled by the tilt angle of the non-rotating pump and motor elements. A tilt angle control apparatus attached to the housing and to the non-rotating pump and motor elements governs that tilt angle.